The present invention relates to a metal halide discharge lamp, a lighting device for a metal halide discharge lamp, and an illuminating apparatus using a metal halide discharge lamp.
A metal halide discharge lamp comprises a light-emitting tube provided with a pair of electrodes arranged to face each other. A rare gas, a halide of a light-emitting metal, and mercury are sealed in the light-emitting tube to form the metal halide discharge lamp. The discharge lamp of the particular construction exhibits a relatively high efficiency and high color rendering properties and, thus, is used widely.
The metal halide discharge lamp is classified into a short arc type and a long arc type. The short arc type metal halide discharge lamp is used in projectors such as a liquid crystal projector in which light rays emitted from a lamp are collected so as to be projected onto a screen, and an overhead projector, and is also used for illumination of shops in the form of down light and spot light. Also, a small short arc type metal halide discharge lamp has come to be used in recent years as a headlamp of a vehicle in place of a halogen lamp.
As described in, for example, Japanese Patent Disclosure (Kokai) No. 2-7347, it is absolutely necessary to seal about 2 to 15 mg of mercury in a metal halide discharge lamp used as a headlamp of a vehicle.
On the other hand, Japanese Patent Disclosure No. 3-112045 discloses a metal halide discharge lamp which does not necessitate the sealing of mercury. In this prior art, a rare gas such as helium or neon is sealed in the lamp at a pressure of 100 to 300 Torr in place of mercury so as to obtain a desired lamp voltage. Since the atom of each of these rare gases has a small radius, the rare gas permeates through a quartz glass and, thus, the hermetic vessel of the lamp is formed of a transparent ceramic material.
On the other hand, the long arc type metal halide discharge lamp is used mainly for the general illumination purposes. For example, the discharge lamp of this type is used as an illumination equipment for a high ceiling, a light projector, a street lamp and as an illumination equipment for roads. Further, a metal halide discharge lamp which generates ultraviolet rays is used for the manufacture of a photo-setting synthetic resin or ink. The metal halide discharge lamp used for this purpose is also of a long arc type.
In any of the short arc type and long arc type metal halide discharge lamps which have been put to practical use nowadays, it is absolutely necessary to use mercury, because, in the metal halide discharge lamp, mercury serves to obtain a desired lamp voltage so as to maintain satisfactory electric properties.
To be more specific, where, for example, the lamp voltage is unduly low, the lamp current must be increased in order to obtain a desired lamp input. In this case, problems are brought about that the current capacities are increased in the related facilities such as the lighting device, illuminating device and wiring. Also increased is the heat generation.
On the other hand, where the lamp current is unduly high, the electrode loss is increased, leading to a low lamp efficiency. To be more specific, the electrode drop of the metal halide discharge lamp is constant for each lamp. As a result, if the lamp voltage is unduly low, the lamp current must be increased for making up for the low lamp voltage, with the result that the electrode loss is increased in proportion to the lamp current so as to lower the lamp efficiency.
As pointed out above, it is generally advantageous in a discharge lamp to set the lamp voltage at a value as close to the input voltage of the lamp as possible, i.e., as high as possible, as far as the arc does not disappear.
Let us describe the reason why the mercury sealing was required in the conventional metal halide discharge lamp while giving consideration to the lamp voltage with reference to FIG. 1. As shown in the drawing, the lamp comprises a hermetic vessel 1, a pair of electrodes 2, 2, and lead wires 3, 3. The lamp voltage V1, which denotes the voltage between the lead wires 3, 3 when the metal halide discharge lamp is lit, can be represented by formula (1) given below:
V1=Exc3x97L+Vdxe2x80x83xe2x80x83(1)
where E is a degree of potential inclination of the plasma between the electrodes, L is a distance between the electrodes, and Vd is an electrode drop.
The potential inclination degree E of the plasma can be represented by formula (2) given below:
E=I/2xcfx80∫"sgr"rdrxe2x80x83xe2x80x83(2)
where I is a lamp current, "sgr" is an electrical conductivity of the plasma, which is a function of temperature T, and r is a distance of an optional point from the center in the radial direction.
If a substance A is supposed to be present within the discharge space during the lighting of the metal halide discharge lamp, the electrical conductivity "sgr" of the substance A at temperature T is given by formula (3) below:
"sgr"=Cxc2x7NE/(Txc2xdxc2x7(NAxc2x7Q))xe2x80x83xe2x80x83(3)
where C is a constant, NE is an electron density, NA is a density of the substance A, and Q is a cross section of impingement of the electron against the substance A.
As apparent from formula (1), the lamp voltage V1 is increased with increase in the potential inclination degree E and with increase in the distance L between the electrodes. On the other hand, formula (2) indicates that the potential inclination degree E is increased with decrease in the electrical conductivity "sgr" and with increase in the lamp current I. Further, formula (3) indicates that the electrical conductivity u is decreased with decrease in the electron density NE and with increase in the density NA of the substance A and in the impinging cross section Q. It follows that, where the distance L between the electrodes and the lamp current I are set constant, the conditions of the substance A, under which the lamp voltage V1 is increased, are that the substance A is unlikely to be ionized to diminish the value NE, that the substance A has a high density within the lamp to increase the value NA, and that substance A has a large cross section Q of the electron impingement.
It should be noted that mercury has a very high vapor pressure, i.e., 1 atmosphere at 361xc2x0 C., is unlikely to be ionized, and has a large cross section of the electron impingement. It follows that a desired lamp voltage can be easily obtained by controlling the sealing amount of mercury in accordance with the size of the lamp. In other words, mercury is sealed in the conventional metal halide discharge lamp because a desired lamp voltage can be obtained easily.
It should be noted in this connection that, in the case of a metal halide discharge lamp, it is necessary to set the mercury vapor pressure higher with miniaturization of the lamp in which the distance L between the electrodes is shortened in order to ensure a desired lamp voltage. For example, in a small short arc type metal halide discharge lamp whose light-emitting tube has an inner volume of 1 cc or less, the mercury vapor pressure during lighting of the lamp is as high as at least 20 atmospheres.
Let us describe the problems which are brought about where mercury is sealed in a metal halide discharge lamp and the problems which are brought about where mercury is not sealed in the conventional metal halide discharge lamp.
Air pollution and water contamination problems attract worldwide attentions nowadays. Since mercury is harmful to the health of the human being, it is naturally desirable to decrease the amount of mercury used or not to use mercury at all in the field of illumination. In other words, the greatest problem inherent in the conventional metal halide discharge lamp is that mercury is sealed in the lamp.
In addition, many problems remain unsolved when it comes to the metal halide discharge lamp in which mercury is sealed for obtaining a desired lamp voltage, as pointed out below:
1. Poor in Rising of Spectral Characteristics in the Start-up Time:
Where a metal halide discharge lamp is used in the headlamp of a vehicle, required is the instant rising of the light flux. To meet this requirement, employed is a lighting system in which xenon is sealed as a starting gas at a high pressure, and a large current is allowed to flow in the initial period of the lighting, followed by gradually decreasing the current with time. It is certainly possible to achieve the instant rising of light flux in this fashion. However, since mercury is rapidly evaporated in the switch-on time, mercury takes much energy so as to cause delay in the rising of the vapor pressure of a light-emitting metal. It follows that mercury continues to emit light of high intensity for 10 to 20 seconds. It should be noted that the light emitted from mercury is poor in color characteristics and in color rendering properties. Also, the chromaticity of the light emitted from mercury fails to fall within a range of whiteness. Since the rising of the spectral characteristics is very poor as described above, it takes a long time to obtain emission of light having desired spectral characteristics.
2. Unsuitable for Light Control (Dimming):
A change in temperature of the light emitting tube brings about a great change in the color temperature of the emitted light and, thus, in the color rendering properties, as apparent from FIG. 2. Specifically, FIG. 2 is a graph showing the distribution of an emission spectral of a conventional short arc type metal halide discharge lamp for projection. The wavelength (nm) is plotted on the abscissa of the graph, with the relative emission power (%) being plotted on the ordinate.
Sealed in the conventional short arc type metal halide discharge lamp are 6.65xc3x97104 Pa of argon as a rare gas, 1 mg of dysprosium iodide (DyI3) as a halide, 1 mg of neodymium iodide (NdI3) as a halide, and 13 mg of mercury. The emission spectral consists of a continuous light emission caused by dysprosium and neodymium and main bright-line spectra caused by the elements given above the arrows in the drawing. As seen from the graph, the bright-line spectrum caused by mercury has a large power.
It should be noted that the amount of light emission from each of the light-emitting metals is changed proportionally to the vapor pressure within the lamp. Since the vapor pressure of a halide of a light-emitting metal is markedly lower than that of mercury, a change in temperature of the light emitting tube causes a change in the evaporation amount of the halide, leading to a change in the vapor pressure within the lamp. As a result, the amount of light emitted from the light-emitting metal is also changed.
On the other hand, the vapor pressure of mercury is so high that a change in temperature of the light emitting tube does not bring about an appreciable change in the mercury vapor pressure, leading to a small change in the amount of light caused by the strong bright-line spectrum of mercury. It follows that, if the input power supplied to the light-emitting tube is decreased, the light emission caused by mercury is rendered relatively predominant. As a result, the color temperature of the emitted light is lowered, leading to poor color rendering properties. This indicates that the conventional metal halide discharge lamp, which requires mercury sealing, is unsuitable for the light control (dimming).
In the case of a headlamp for a vehicle, dimming is required for the lighting in the day time (day light) employed in the U.S.A. and Europe. However, the color characteristics are markedly impaired in the conventional metal halide discharge lamp requiring the mercury sealing.
3. Large Unevenness in Properties:
The metal halide discharge lamps having mercury sealed therein are uneven in temperature of the light emitting tubes, which are caused by unevenness in the size of the individual lamps. As a result, unevenness in the characteristics tends to be brought about even under the same input power. Also, the characteristics are likely to be changed by the temperature elevation in the coolest region caused by the blackening of the light-emitting tube used over a long period of time. These difficulties tend to bring about a problem particularly where a plurality of metal halide discharge lamps are used in combination for illumination as in, for example, shops.
4. Difficult to Re-start up Instantly:
As described previously, the distance between the paired electrodes is small in a short arc type small metal halide discharge lamp, making it necessary to set the mercury vapor pressure at a high value. Specifically, the mercury vapor pressure is set at such a high value as, for example, at least 20 atmospheres.
Further, when used in a headlamp for a vehicle, xenon is also sealed in the lamp at a high pressure. For example, the xenon pressure is as high as about 35 atmospheres during the lighting. Since the mercury vapor pressure and the xenon vapor pressure are very high during the lighting, it is necessary to apply a pulse voltage of a very large power in the re-start up time. It follows that the lighting circuit is rendered costly. In addition, it is necessary to insulate the circuit, the lamp and the equipment housing them against a high voltage.
5. Rupture of Light-Emitting Tube
Since the mercury vapor pressure is very high during the lighting as described previously, strain of the lamp is increased during lighting of the lamp over a long period of time, with the result that the lamp tends to be ruptured. The problem of rupture markedly lowers the reliability of the lamp.
6. Low Screen Brightness when used in Projector:
Where a short arc type metal halide discharge lamp is used in a projector in which the light emitted from the lamp used as a light source is collected through an optical system such as a liquid crystal projector so as to illuminate, for example, a screen arranged apart from the projector, it is of high importance to suppress the loss of the light emitted from the discharge lamp when the emitted light passes through the optical system so as to permit the emitted light to arrive at the screen as much as possible. In order to improve the brightness of the screen by suppressing the light loss, it is necessary for the arc of the discharge lamp to be narrowed thin. The expression xe2x80x9cnarrow arcxe2x80x9d denotes that the arc temperature distribution is sharp.
It should be noted in this connection that the light emitted from mercury is absorbed and, thus, is optically thick. Since energy is absorbed by the absorption of the light emitted in the intermediate and low temperature regions, the temperature is elevated. As a result, the arc temperature is distributed to depict a parabola, making it difficult to narrow the arc. On the other hand, it is known to the art that, if the light emission is very much increased by using scandium or a rare earth metal as a light-emitting metal, the arc can be narrowed even in the presence of mercury. In this case, however, convection occurs vigorously if the lighting pressure of mercury is high, with the result that the arc is rendered unstable. It follows that it is impossible to put this technique to practical use.
In the metal halide discharge lamp which does not necessitate mercury sealing, the partial pressure of helium or neon within the light-emitting tube is markedly increased during the lighting. If the light-emitting tube is constructed to withstand the high pressure, it is certainly possible to obtain a metal halide discharge lamp in which mercury is not sealed. The possibility itself of obtaining a metal halide discharge lamp not requiring mercury sealing is worthy of a favorable evaluation. However, it is practically difficult to permit a metal halide discharge lamp of the construction similar to that of the conventional lamp to withstand the high pressure within the lamp during the lighting of the lamp. Where, for example, a lamp voltage of 50 to 60V is required in s small metal halide discharge lamp, the pressure of helium or neon within the lamp is expected to exceed 150 atmospheres during the lighting of the lamp. It follows that the hermetic vessel widely used in the conventional lamp fails to obtain a high reliability in respect of the measure against rupture of the hermetic vessel.
A main object of the present invention is to provide a metal halide discharge lamp which essentially permits disusing mercury, which is harmful to the living environment of the human being, and which also permits obtaining electrical characteristics and light-emitting characteristics substantially equal to those produced by a metal halide discharge lamp in which is sealed mercury, to provide a lighting device for the particular metal halide discharge lamp, and to provide an illumination apparatus using the particular metal halide discharge lamp.
An auxiliary object of the present invention is to provide a metal halide discharge lamp which permits a good rising of chromaticity at the start-up time, which permits light control for the dimming purpose, which is small in unevenness of the characteristics, which permits an instant re-start up easily, and which is effective for preventing the hermetic vessel from being ruptured, to provide a lighting device for the particular metal halide discharge lamp, and to provide an illumination apparatus using the particular metal halide discharge lamp.
Another auxiliary object of the present invention is to provide a metal halide discharge lamp in which mercury is not sealed, which is small in heat loss, and which is effective for preventing the light emitting efficiency from being lowered, to provide a lighting device for the particular metal halide discharge lamp, and to provide an illumination apparatus using the particular metal halide discharge lamp.
Another auxiliary object of the present invention is to provide a metal halide discharge lamp which is lit by a DC current to emit light free from color difference and color separation, and which permits the lighting circuit to be manufactured as a low cost, to provide a lighting device for the particular metal halide discharge lamp, and to provide an illumination apparatus using the particular metal halide discharge lamp.
Another auxiliary object of the present invention is to provide a metal halide discharge lamp which is adapted for use as a headlamp for a vehicle such as an automobile, to provide a lighting device for the particular metal halide discharge lamp, and to provide an illumination apparatus using the particular metal halide discharge lamp.
Still another auxiliary object of the present invention is to provide a practical metal halide discharge lamp which is effective for preventing a hermetic vessel from being ruptured during the lighting of the lamp even if the mechanical strength of the hermetic vessel is substantially equal to that of the conventional hermetic vessel in which is sealed mercury, to provide a lighting device for the particular metal halide discharge lamp, and to provide an illumination apparatus using the particular metal halide discharge lamp.
According to a first aspect of the present invention, there is provided a metal halide discharge lamp which essentially permits disusing mercury, comprising:
a refractory and transparent hermetic vessel;
a pair of electrodes fixed to the hermetic vessel; and
a discharge medium sealed in the hermetic vessel and containing a first halide, a second halide and a rare gas, the first halide being a halide of a metal which achieves a desired light emission, the second halide having a relatively high vapor pressure, being at least one halide of a metal which is unlikely to emit a visible light compared with the metal of the first halide, and acting as a buffer gas.
The terms used in the present specification are defined to denote the technical meanings described below unless otherwise specified particularly:
Hermetic Vessel . . . The refractory and transparent hermetic vessel used in the present invention is formed of a material capable of fully withstanding the ordinary operating temperature of a discharge lamp and also capable of leading the visible light of a desired wavelength region emitted by the discharge to the outside. Any material can be used for forming the hermetic vessel as far as the requirements given above are met. For example, it is possible to use quartz glass, or ceramic material such as transparent alumina or YAG as well as single crystals thereof.
Incidentally, it is acceptable in the present invention to form on the inner surface of the hermetic vessel a transparent film resistant to halogen and to metals, as required. It is also acceptable to modify the inner surface of the hermetic vessel, as required.
Electrode . . . The metal halide discharge lamp of the present invention can be constructed so as to be lit by either an AC current or a DC current. Therefore, where the lamp is operated by an AC current, the paired electrodes are of the same construction. However, where the lamp is operated by a DC current, the anode whose temperature is elevated severely in general is allowed to have a heat radiating area larger than that of the cathode.
The metal halide discharge lamp of the present invention may be either of a short arc type or of a long arc type. The short arc type discharge lamp is of a so-called electrode stability type, in which the distance between the electrodes arranged within the hermetic vessel is diminished so as to allow the electrodes to stabilize the arc discharge. Therefore, the light emission from the discharge lamp can be made as close to that of a dot light source as possible, making it possible to allow the optical system such as a light reflector or a lens to collect light efficiently. A small short arc type metal halide discharge lamp is used in a projector such as a liquid crystal projector or in a headlamp for a vehicle such as an automobile. In this case, a suitable distance between the electrodes of the metal halide discharge lamp is practically at most 6 mm. If the distance between the electrodes exceeds 6 mm, the light emission from the discharge lamp is rendered widely different from that from a dot light source, leading to poor focusing characteristics of the optical system. Where, for example, the discharge lamp, in which the distance between the electrodes exceeds 6 mm, is used as a light source of a liquid crystal projector, the screen brightness is lowered.
Under the circumstances, the small short arc type metal halide discharge lamp of the present invention is defined to denote a discharge lamp in which the distance between the electrodes is at most 6 mm. Preferably, the distance between the electrodes should be at most 5 mm. Further, where the discharge lamp is used in a projector such as a liquid crystal projector, the distance between the electrodes should be 1 to 3 mm. Incidentally, the distance in question represents the distance between the tips of the electrodes.
On the other hand, the long arc type metal halide discharge lamp of the present invention is of a so-called tube wall stability type, in which the distance between the electrodes arranged within the hermetic vessel is set longer than the inner diameter of the hermetic vessel so as to allow the arc discharge to be stabilized by the inner surface of the hermetic vessel. In general, the long arc type metal halide discharge lamp is widely used for the illumination purpose.
Discharge Medium . . . As described previously, the discharge medium used in the present invention consists essentially of a first halide, a second halide and a rare gas.
The first halide is a halide of a metal which emits a desired light such as a visible light or an ultraviolet light. Where a halide of a metal which efficiently emits a visible light is used as the first halide in order to utilize the visible light, the vapor pressure of the first halide is not necessarily high in general during the lighting of the lamp.
The second halide, which is also a halide of a metal, should have a relatively high vapor pressure during the lighting of the lamp. The metal contained in the second halide is not particularly limited as far as the metal is unlikely to emit a visible light compared with the metal contained in the first halide. The expression xe2x80x9crelatively high vapor pressurexe2x80x9d denotes that the vapor pressure need not be as high as the vapor pressure of mercury. Preferably, the pressure within the hermetic vessel during the lighting of the lamp should be about 5 atmospheres or less. During operation of the lamp, it is desirable for the vapor pressure of the second halide to be at least about 10 times as high as that of the first halide in the lowest temperature region of the lamp. Further, the metal contained in the second halide should be less likely to emit a visible light than the metal contained in the first halide. This implies that, though the metal contained in the second halide may slightly emit a visible light, the visible light emission from the particular metal is relatively small.
It should be noted in this connection that the light emitted from Fe or Ni contains components having wavelengths of an ultraviolet region in an amount larger than that of the components having wavelengths of a visible region. On the other hand, the light emitted from Ti, Al or Zn contains a large amount of components having wavelengths of a visible region. If Ti, Al or Zn is excited singly to emit light, the energy is concentrated on the particular metal so as to emit a large amount of visible light. However, if the metal contained in the second halide has an energy level higher than that of the metal contained in the first halide so as to allow the metal contained in the second halide to be less likely to emit light, the energy is concentrated on the light emission from the first halide under the condition that the first and second halides are present together, with the result that the light emission from the metal contained in the second halide is suppressed.
The vapor of the second halide functions basically as a buffer gas during the lighting of the lamp like mercury used in the conventional metal halide discharge lamp. Table 1 exemplifies various second halides used in the present invention and the temperatures at which the vapor pressures of these second halides reach one atmosphere. Incidentally, slight differences are seen depending on the literature in the temperatures at which the vapor pressures of these second halides reach one atmosphere. However, the values given in Table 1 are considered to be substantially correct.
Almost all the halides shown in Table 1 have a vapor pressure lower than that of mercury. Also, the control range of the lamp voltage is narrower than that for mercury. However, the control range of the lamp voltage can be widened by sealing a plurality of the second halides in combination in the hermetic vessel, as required. For example, where AlI3 is under the state of incomplete evaporation and a desired lamp voltage is not obtained, the lamp voltage remains unchanged even if an additional AlI3 is sealed in the hermetic vessel.
On the other hand, if ZnI2 is sealed in the hermetic vessel in place of the additional AlI3, the lamp voltage can be increased because the lamp voltage produced by the function of ZnI2 is added to the lamp voltage produced by the initially sealed AlI3. Further, if another second halide is added, a higher lamp voltage can be obtained.,
It should also be noted that it is not absolutely necessary for the second halide not to emit a visible light. It is acceptable for the second halide to emit a visible light, if the ratio of the emitted visible light to all the visible light emitted from the discharge lamp is small enough to make the effect sufficiently small, i.e., the effect given by the visible light emitted from the second halide to all the visible light emitted from the discharge lamp.
Further, a third halide can also be sealed in the hermetic vessel in the present invention in addition to the first and second halides in order to allow the third halide to, for example, correct the arc temperature distribution so as to suppress the heat loss.
Halogen . . . It is most desirable in terms of the reactivity to use iodine as a halogen element contained in each of the first and second halides. It is also possible to use bromine, chlorine and fluorine as a halogen element, which exhibit strong reactivity in the order mentioned. In short, any of iodine, bromine, chlorine and fluorine can be used in the present invention. Further, it is possible to use different halogen compounds in combination. For example, a iodide and a bromide can be used together.
A rare gas, which functions as a starting gas and a buffer gas, is also sealed in the hermetic vessel in the present invention. Any kind of the rare gas can be used in the present invention as far as the rare gas does not permeates through the hermetic vessel. It should be noted in this connection that neon is likely to permeate through a quartz glass. Naturally, where the hermetic vessel is made of a quartz glass, it is desirable to use argon, krypton or xenon as a rare gas sealed in the hermetic vessel.
If the rare gas is sealed at a high pressure, it is possible to improve the rising characteristics of the light flux emitted from the metal halide discharge lamp. Good rising characteristics of the light flux are desirable for what purpose the discharge lamp may be used, and are particularly important where the discharge lamp is used in, for example, a headlamp of a vehicle such as an automobile and in a liquid crystal projector.
Mercury . . . The metal halide discharge lamp of the present invention permits xe2x80x9cessentiallyxe2x80x9d disusing mercury. In other words, mercury is not sealed at all in the hermetic vessel. However, it is acceptable for mercury to be present inside the hermetic vessel in an amount of less than 0.3 mg/cc, preferably, not more than 0.2 mg/cc, of the inner volume of the hermetic vessel. Naturally, in terms of the living environment of the human being, it is desirable for mercury not to be sealed at all in the hermetic vessel of the discharge lamp. Where the electric characteristics of the discharge lamp are maintained by the mercury vapor as in the prior art, mercury is sealed in an amount of at least 20 mg/cc of the inner volume of the hermetic vessel in the case of a short arc type metal halide discharge lamp, and in an amount of at least 5 mg/cc of the inner volume of the hermetic vessel in the case of a long arc type metal halide discharge lamp. Compared with the mercury amount required in the prior art, it is considered reasonable to state that the metal halide discharge lamp of the present invention permits xe2x80x9cessentiallyxe2x80x9d disusing mercury.
Function . . . As described above, the discharge medium used in the present invention comprises as a first halide a halide of a metal which mainly contributes to the emission of a desired light, and as a second halide a halide of a metal which is unlikely to emit a visible light compared with the metal contained in the first halide. In the present invention, the second halide is essentially substituted for mercury used in the conventional metal halide discharge lamp. As a result, the lamp voltage is determined by mainly the amount of evaporation of the second halide in the present invention. Also, the vapor pressure of the halide is determined by the temperature in the coolest region of the hermetic vessel.
The vapor pressure of the second halide during the lighting, which is lower than that of mercury but is clearly higher than that of the first halide, should be at most about 5 atmospheres.
Under the circumstances, the metal halide discharge lamp of the present invention performs a desired function without requiring a substantial mercury sealing, and permits obtaining a lamp voltage sufficient for obtaining the electric characteristics and light-emitting characteristics substantially equal to those of the conventional lamp requiring a mercury sealing. Incidentally, the term xe2x80x9csubstantiallyxe2x80x9d given above denotes that, in the present invention, it is acceptable for the obtained electrical and light-emitting characteristics to be somewhat inferior to those obtained in the prior art within a practically workable range. The slight difference in question gives rise to no practical problem in view of the fact that it is possible for the metal halide discharge lamp of this type to be lit by an electronic lighting device. However, the lamp voltage can be further increased in the present invention by applying a heat insulating means to the hermetic vessel, as required.
In the present invention, the hermetic vessel is substantially free from the mercury sealing, and the visible light emission is achieved substantially solely by the metal contained in the first halide. As a result, the metal halide discharge lamp of the present invention permits a good rising of chromaticity in the start-up time. Even where the input to the lamp is changed, the changes in the color temperature and the color rendering properties of the emitted light can be suppressed, making it possible to achieve light control (dimming).
It should also be noted that, in the metal halide discharge lamp of the present invention, the lamp characteristics are less affected by the unevenness in the size and shape of the lamps, making it possible to suppress the unevenness in the color of the emitted light.
Further, an instant re-start up can be achieved easily in the present invention because the vapor pressure of the second halide is clearly lower than that of mercury in almost all the cases. It follows that the height of the starting pulse voltage applied for the re-start up can be lowered. As a result, it is possible to lower the dielectric strength of the lighting device, start-up device, wiring and illumination apparatus, leading to a low manufacturing cost.
Further, the vapor pressure during the lighting is not extremely high in the present invention. To be more specific, it is of no difficulty to lower the vapor pressure in question to about 60% of the value required in the step of sealing mercury, making it possible to prevent substantially completely the hermetic vessel from being ruptured during the lighting of the lamp.
Still further, the metal halide discharge lamp of the present invention permits somewhat improving the color rendering properties on the basis that the light emitting efficiency is substantially the same.
As described above, each of the short arc type and long arc type metal halide discharge lamp of the present invention exhibits the steady state characteristics substantially equal to those exhibited by the prior art.
In addition, the technical idea of the present invention.can be applied over a wide range of the lamp power ranging between scores of W and several kW.
The metal halide discharge lamp of the present invention may be either of a single tube type, in which the hermetic vessel is exposed directly to the outer atmosphere, or a double wall type, in which the hermetic vessel is inserted into an outer tube. Each of these single tube type and double wall type lamps of the present invention produces a desired function and effect. Further, if the inner space of the outer tube is held at a vacuum condition in the double wall type metal halide discharge lamp, the heat loss can be suppressed so as to further improve the light emitting efficiency.
What should also be noted is that, where the technical idea of the present invention is applied to a short arc type metal halide discharge lamp, it is desirable to construct the lamp to narrow the arc, as described previously. If the arc is narrowed, the light collecting efficiency can be improved. It follows that a marked improvement in brightness can be obtained, if the short arc type lamp of the particular construction is used in, for example, a headlamp of a vehicle such as an automobile in combination with a reflector, or in an optical system of the reflector as in an illumination apparatus for shops or optical fiber illumination apparatus.
According to a second aspect of the present invention, there is provided a metal halide discharge lamp, comprising:
a refractory and transparent hermetic vessel;
a pair of electrodes mounted within the hermetic vessel; and
a discharge medium sealed in the hermetic vessel and containing a first halide, a second halide and a rare gas, the first halide being a halide of at least one metal selected from the group consisting of sodium, scandium and a rare earth metal, and the second halide being having a relatively high vapor pressure, and being at least one halide of a metal which is unlikely to emit a visible light compared with the metal of the first halide.
In the second aspect of the present invention, a visible light is emitted from the first halide. In addition, the metal halides suitable for the various general uses are specified in the second aspect in view of the light emitting efficiency and the color rendering properties. It is possible to use a single compound or a plurality of compounds in combination as the first halide.
The hermetic vessel included in the metal halide discharge lamp of the second aspect is essentially free from mercury to enable the present invention of the second aspect to produce the function and effect similar to those produced by the invention of the first aspect.
According to a third aspect of the present invention, the discharge medium used in the metal halide discharge lamp of the first or second aspect of the present invention contain a halide of cesium. Where a halide of cesium is sealed in the hermetic vessel, the temperature distribution of the arc is rendered flat so as to diminish the temperature gradient, as in the case of sealing mercury, with the result that the heat loss in the light emitting tube is decreased. It follows that the light emitting efficiency is increased to reach a level close to the case of sealing mercury, compared with the case where a halide of cesium is not sealed.
To be more specific, cesium generated by decomposition of the cesium halide in the arc has a low ionization voltage and, thus, tends to be ionized to release an electron even in the intermediate temperature region of the arc, which is a relatively low temperature region of the arc. It follows that the presence of cesium in the arc causes the electron concentration to be increased in the intermediate temperature region of the arc.
It should be noted that the electrical conductivity "sgr" is proportional to the electron density. Since the energy input in a certain temperature region is represented by "sgr" E2, where E denotes the intensity of the electric field, the input energy is increased with increase in the electrical conductivity "sgr", that is increase in the electron density. In conclusion, where a halide of cesium is sealed in the hermetic vessel, the energy input is increased in the intermediate temperature region, with the result that the temperature in the intermediate temperature region of the arc is elevated.
On the other hand, since the total input to the metal halide discharge lamp is constant, the temperature in the high temperature region of the arc is relatively lowered, as apparent from the energy balance. It follows that the temperature distribution of the arc is rendered flat so as to diminish the temperature gradient in the case of sealing a halide of cesium, as in the case of sealing mercury.
On the other hand, in the conventional metal halide discharge lamp in which is sealed mercury, mercury also emits light. However, the light emitting efficiency of mercury itself is not sufficiently high, as pointed out previously. In the present invention, however, the hermetic vessel is essentially free from mercury. Therefore, a metal exhibiting a light emitting efficiency higher than that of mercury, e.g., scandium or sodium, is sealed in the hermetic vessel so as to provide a metal halide discharge lamp exhibiting a high light emitting efficiency.
Further, disuse of mercury permits producing a function and effect similar to those produced by the metal halide discharge lamp according to the first aspect of the present invention.
A metal halide discharge lamp according to a fourth aspect of the present invention comprises an outer tube housing the light emitting tube and a heat insulating means for suppressing the loss of heat generated from the light emitting tube in addition to the metal halide discharge lamp according to the first aspect of the present invention. Since the loss of heat generated from the light emitting tube is suppressed by the heat insulating means, the light emitting efficiency of the metal halide discharge lamp is improved. The heat insulating means may be of any construction as far as it is possible to suppress the loss of heat generated from the light emitting tube. For example, the heat insulating means can be constructed as follows.
Specifically, the inner space of the outer tube is held at a vacuum condition so as to suppress convection and conduction of the heat generated from the light emitting tube. It follows that the heat loss is suppressed so as to keep the discharge medium warm. In this case, the heat insulating means may be of any desired specific construction and shape. Also, any desired material can be used for forming the heat insulating means. In the present invention, the inner space of the outer tube is defined to be held at a vacuum condition. To be more specific, the inner pressure of the outer tube is at most 1.33xc3x97103 Pa.
It is desirable in the metal halide discharge lamp of the present invention to use a heat ray reflecting-visible light transmitting film which reflects the heat ray radiated from the light emitting tube to the outside back into the light emitting tube and which transmits a visible light. Use of the particular film permits decreasing the heat loss accompanying the radiation so as to keep the discharge medium warm. The heat reflecting-visible light transmitting film can be formed on the inner and/or outer surfaces of a cylindrical body made of quartz glass and interposed between the light emitting tube and the outer tube, on the inner and/or outer surfaces of the outer tube, and on the outer surface of the light emitting tube.
Needless to say, the particular means described above can be used in combination appropriately.
Since the discharge lamp of the present invention comprises a heat insulating means for suppressing the loss of heat generated from the light emitting tube, the loss of heat generated by the discharge within the light emitting tube can be suppressed so as to lower the heat loss of the light emitting tube. It follows that the light emitting efficiency is improved.
Further, the hermetic vessel included in the metal halide discharge lamp according to the fourth aspect of the present invention is substantially free from mercury, making it possible to obtain a function and effect similar to those produced by the invention of the first aspect.
According to a fifth aspect of the present invention, there is provided a metal halide discharge lamp which essentially permits disusing mercury and which is lit by a DC current, comprising;
a refractory and transparent hermetic vessel;
an anode and a cathode fixed to the hermetic vessel; and
a discharge medium sealed in the hermetic vessel and containing a first halide, a second halide and a. rare gas, the first halide being a halide of at least one metal selected from the group consisting of sodium, scandium, and a rare earth metal, and the second halide having a relatively high vapor pressure and being a halide of at least one metal which is unlikely to emit a visible light compared with the metal of the first halide.
If the conventional metal halide discharge lamp having mercury sealed therein is lit by a DC current, the light emitting metal, e.g., sodium or scandium, is positively ionized and, thus, sucked toward the cathode, with the result that the concentration of the light emitting metal on the anode side is rendered lower than that on the cathode side. Also, mercury is somewhat sucked toward the cathode. However, since mercury is originally sealed in a predominantly large amount, a sufficiently large amount of mercury is present on the anode side, too. As a result, the light emission from the light emitting metal is rendered markedly weak on the anode side to cause light to be emitted from mainly mercury on the anode side, though a sufficiently large amount of light emission from the light emitting metal can be obtained on the cathode side. It follows that a marked color separation is brought about between the two electrodes to make the discharge lamp unsuitable for the practical use. Under the circumstances, where the color separation should be avoided, a metal halide discharge lamp requiring the mercury sealing is used exclusively for the case where the discharge lamp is lit by an AC current.
In the present invention, however, the difference in color temperature between the electrodes is small so as to permit the discharge lamp to be put to practical use in spite of the fact that the second halide is substantially substituted for mercury, and the discharge lamp is lit by a DC current. It should be noted in this connection that, since the second halide is unlikely emit a visible light, the metal contained in the first halide emits light of high intensity even on the anode side in the present invention, leading to the small difference in color temperature between the two electrodes.
It should also be noted that, in a headlamp for a vehicle such as an automobile or in a lamp for a liquid crystal projector, used is an electronic lighting device including a metal halide discharge lamp. Where an AC current is used for lighting the discharge lamp, a DC current generated from a battery power source or a DC current obtained by rectifying an AC current of a commercial frequency is converted in general into a high frequency AC current and, then, supplied to the metal halide discharge lamp.
In the present invention, however, the metal halide discharge lamp is constructed to be adapted for the lighting with a DC current, making it unnecessary to convert the DC current into a high frequency AC current. This makes it possible to simplify the construction of the electronic circuit for lighting the discharge lamp so as to make the lighting device small in size, light in weight and low in manufacturing cost.
Further, since.mercury is not used, the metal halide discharge lamp according to the fifth aspect of the present invention produces the function and effect similar to those obtained in the invention of the first aspect.
According to a sixth aspect of the present invention, there is provided a metal halide discharge lamp which permits essentially disusing mercury and which is used in a headlamp having a rated power of at most 100 W, comprising:
a refractory and transparent hermetic vessel;
a pair of electrodes fixed to the hermetic vessel; and
a discharge medium sealed in the hermetic vessel and containing a first halide, a second halide and a rare gas, the first halide being a halide of at least one metal selected from the group consisting of sodium, scandium, and a rare earth metal, and the second halide having a relatively high vapor pressure and being a halide of at least one metal which is unlikely to emit a visible light compared with the metal of the first halide.
A metal halide discharge lamp which is used in a headlamp having a rated power of at most 100 W is featured in that the distance between the two electrodes is small and the tube wall load of the discharge lamp is heavy. Therefore, when it comes to the conventional metal halide discharge lamp requiring the mercury sealing, a such a high mercury vapor pressure as at least 20 atmospheres is required in order to obtain a desired lamp voltage, as described previously. As a result, the hermetic vessel tends to be broken relatively easily.
It is also necessary to seal xenon at a high pressure in order to improve the rising characteristics of the light flux. Specifically, the xenon pressure during the lighting is as high as about 35 atmospheres. It follows that, in order to perform starting by breaking down the starting gas, it is necessary to apply a starting pulse voltage of a high voltage and a large power. Since a higher starting pulse voltage is required for the instant start up, it is necessary to increase the grade of the dielectric strength of the lighting circuit, illumination apparatus and wiring to conform with the high starting pulse voltage, leading to a high manufacturing cost.
As described above, it is certainly possible to solve the problem in respect of the rising characteristics of the light flux by sealing xenon at a high pressure, by applying a high starting pulse voltage, and by supplying a large current immediately after the light, followed by gradually decreasing the current supply. However, the conventional metal halide discharge lamp remains to be unsatisfactory in the rising characteristics of chromaticity. To be more specific, xenon emits light first and, then, mercury emits light. The light emission from mercury is continued for 10 to 20 seconds. What should be noted is that the light emitted from mercury is poor in color rendering properties, failing to fall even within a range of white color.
In the present invention, however, mercury is not sealed in the hermetic vessel, making it possible to reduce the inner pressure of the hermetic vessel to about 60% of the conventional level. It follows that it is possible to alleviate markedly the problems inherent in the prior art in respect of the breakage of the hermetic vessel and the starting pulse voltage.
In the sixth aspect of the present invention, the first halide is limited to a halide of at least one metal selected from the group consisting of sodium and scandium. The particular limitation is effective for obtaining emission of white light required in a headlamp at a very high light emitting efficiency.
Further, since mercury is not sealed in the hermetic vessel, a function and effect similar to those obtained in the first aspect of the present invention can also be obtained in the sixth aspect. It follows that the metal halide discharge lamp according to the sixth aspect of the present invention is quite suitable for use in a headlamp for a vehicle.
According to a seventh aspect of the present invention, the second halide used in the metal halide discharge lamp according to the fifth or sixth aspect of the present invention is limited to a halide of at least one metal selected from the group consisting of magnesium, iron, cobalt, chromium, zinc, nickel, manganese, aluminum, antimony, beryllium, rhenium, gallium, titanium, zirconium and hafnium. In other words, metals suitable for forming the second halide are specified in the seventh aspect of the present invention.
According to an eighth aspect of the present invention, the second halide used in the metal halide discharge lamp according to any of the first to third aspects or any of the fifth to seventh aspects of the present invention is limited to a halide of at least one metal selected from the group consisting of iron, zinc, manganese, aluminum, and gallium.
In other words, metals most suitable for forming the second halide are specified in the eighth aspect of the present invention. It should be noted in this connection that the metal halides specified in the eighth aspect are most suitable for use as the main component of the second halide. What should be noted is that a further improved lamp voltage can be obtained, if a halide of at least one metal selected from the group consisting of magnesium, cobalt, chromium, nickel, antimony, beryllium, rhenium, titanium, zirconium and hafnium is used as an auxiliary component of the second halide together with the main component specified in the eighth aspect.
According to a ninth aspect of the present invention, the second halide used in the metal halide discharge lamp according to any of the first to third aspects or any of the fifth to eighth aspects of the present invention is sealed in the hermetic vessel in an amount of 0.05 to 200 mg/cc of the inner volume of the hermetic vessel.
In the ninth aspect of the present invention, specified is a general range of the sealing amount of the second halide. Depending on the specific halide selected as the second halide, a suitable range of the sealing amount is narrower than noted above. However, the range of the sealing amount noted above should be taken as a generally satisfactory range.
In a tenth aspect of the present invention, the second halide used in the metal halide discharge lamp according to the sixth aspect of the invention is defined to be sealed in the hermetic vessel in an amount of 1 to 200 mg/cc of the inner volume of the hermetic vessel. Defined in the tenth aspect is a suitable sealing amount of the second halide, covering the case where the metal halide discharge lamp is used in a headlamp for a vehicle.
According to an eleventh aspect of the present invention, the rare gas sealed in the hermetic vessel of the metal halide discharge lamp according to the first to tenth aspects of the present invention is defined to be sealed at a pressure of at least one atmosphere. The eleventh aspect is to increase the pressure of the rare gas sealed in the hermetic vessel so as to improve the rising characteristics of the light flux. The good rising characteristics of the light flux, which are desirable for any use of the discharge lamp, are particularly desirable where the discharge lamp is used in a headlamp of a vehicle. It is desirable to use a small short arc type metal halide discharge lamp where the discharge lamp is used in a headlamp for a vehicle.
In a twelfth aspect of the present invention, the rare gas sealed in the hermetic vessel of the metal halide discharge lamp according to the sixth to tenth aspects of the present invention is defined to be sealed at a pressure of 1 to 15 atmospheres. Defined in the twelfth aspect is a rare gas sealing pressure suitable for the case where the metal halide discharge lamp is used in a headlamp of a vehicle.
According to a thirteenth aspect of the present invention, the hermetic vessel included in the metal halide discharge lamp according to the sixth, tenth or twelfth aspects of the present invention is defined to have an inner diameter of 3 to 10 mm and an outer diameter of 5 to 13 mm. Defined in the thirteenth aspect is the size of the hermetic vessel suitable for the case where the metal halide discharge lamp is used in a headlamp of a vehicle.
According to a thirteenth aspect of the present invention, the hermetic vessel included in the metal halide discharge lamp according to the sixth, tenth or twelfth aspects of the present invention is defined to have an inner diameter of 3 to 10 mm and an outer diameter of 5 to 13 mm. Defined in the thirteenth aspect is the size of the hermetic vessel suitable for the case where the metal halide discharge lamp is used in a headlamp of a vehicle.
According to a fourteenth aspect of the present invention, the distance between the two electrodes mounted in the hermetic vessel included in the metal halide discharge lamp according to the sixth, tenth, twelfth and thirteenth aspects of the present invention is defined to be 1 to 6 mm. Defined in the fourteenth aspect is the distance between the two electrodes mounted in the hermetic vessel, the distance being suitable for the case where the metal halide discharge lamp is used in a headlamp of a vehicle. If the distance between the electrodes exceeds 6 mm, the discharge lamp fails to act as a dot light source, leading to a low light collecting effect. More preferably, the distance between the electrodes should fall within a range of between 1 and 5 mm.
According to a fifteenth aspect of the present invention, the metal halide discharge lamp according to the sixth, tenth, and twelfth to fourteenth aspects of the present invention is defined to be constructed so as to be lit by a DC current. Defined in the fifteenth aspect is the discharge lamp is lit by a DC current so as to miniaturize the lighting device when the discharge lamp is used in a headlamp for a vehicle and to lower the manufacturing cost of the discharge lamp. To be more specific, a battery power source is generally mounted to a vehicle such as an automobile. Therefore, use of a lighting device utilizing a DC current permits simplifying the circuit construction, compared with the case where a DC current is converted first into an AC current and, then, supplied to the metal halide discharge lamp so as to light the discharge lamp. The particular effect remains unchanged even in the case where a control means such as a voltage increasing chopper or a voltage decreasing chopper is used for controlling the DC power to have a desired voltage, because the particular control means is used, when required, even in the case of using an AC power for lighting the discharge lamp. In the present invention, mercury is not sealed in the hermetic vessel, with the result that the color separation problem need not be worried about in practice. It follows that it is possible to use a DC current for lighting the discharge lamp of the present invention.
According to a sixteenth aspect of the present invention, the discharge medium used in the metal halide discharge lamp according to the sixth, tenth, and twelfth to fifteenth aspects of the present invention is defined to contain a halide of cesium. In the sixteenth aspect of the present invention, a halide of cesium is sealed in the hermetic vessel included in the metal halide discharge lamp used in a headlamp for a vehicle so as to flatten the arc gradient and, thus, to improve the light emitting efficiency. As a matter of fact, the light emitting efficiency of the discharge lamp according to the sixteenth aspect of the present invention is higher than that of the conventional metal halide discharge lamp having mercury sealed therein.
According to a seventeenth aspect of the present invention, the metal halide discharge lamp according to the sixth, tenth and twelfth to sixteenth aspects of the present invention is defined to further comprise an outer tube having the hermetic vessel housed therein and having the inner space kept at a vacuum condition. In the seventeenth aspect of the present invention, the hermetic vessel included in the metal halide discharge lamp used in a headlamp of a vehicle is housed in the outer tube having the inner space held at a vacuum condition, with the result that the light emitting efficiency of the discharge lamp is rendered higher than that of the conventional discharge lamp having mercury sealed therein.
According to an eighteenth aspect of the present invention, the metal halide discharge lamp according to the sixth, tenth, and twelfth to seventeenth aspects of the present invention further comprises means for removing ultraviolet light which permits xe2x80x9csubstantiallyxe2x80x9d removing an ultraviolet light from the light led to the outside. The expression xe2x80x9csubstantialxe2x80x9d removal denotes that the ultraviolet light is removed to a practically allowable level. In other words, the substantial removal does not necessarily imply that 100% of the ultraviolet light is removed.
The ultraviolet light removing means may be of any construction as far as the ultraviolet light is substantially removed. For example, the light emitting tube is housed in an outer tube made of a glass material of the composition capable of removing the ultraviolet light. Incidentally, it is possible for the outer tube to communicate with the outer atmosphere. Alternatively, the outer tube may be hermetic and may have the inner space held at a vacuum condition.
It is also possible to impart an ultraviolet light removing function to the inner surface of the light emitting tube or to the light emitting tube itself. To be more specific, the ultraviolet light shielding function can be imparted by converting the material texture of the inner or outer surface of the light emitting tube into an ultraviolet light shielding texture or by forming a transparent film capable of shielding an ultraviolet light on the inner or outer surface of the light emitting tube. Further, a pair of ultraviolet light shielding cylinders may be arranged outside the light emitting tube.
Since the ultraviolet light led to the outside is substantially removed in the present invention, the headlamp is prevented from being deteriorated by the ultraviolet light. Also, the human eyes are prevented from being irradiated with an ultraviolet light. Further, in the case of using an outer tube, the hermetic vessel is mechanically protected by the outer tube.
According to an additional aspect of the present invention, there is provided a lighting device for a metal halide discharge lamp, comprising:
a metal halide discharge lamp defined in any one of the sixth, tenth and twelfth to eighteenth aspects of the present invention; and
a lighting circuit constructed to supply current in an amount at least three times as much as a rated lamp current immediately after the lighting of the metal halide discharge lamp, followed by decreasing the current with time.
Defined in this additional aspect of the present invention is a lighting device for a metal halide discharge lamp which meets the rising characteristics of the light flux required for a headlamp for a vehicle. The lighting circuit may be operated by either an AC current or a DC current. Also, the lighting circuit may be of any desired construction as far as the requirements given above are satisfied.
Further, according to still additional aspect of the present invention, there is provided an illumination apparatus, comprising:
an illumination apparatus body; and
a metal halide discharge lamp defined in any one of the first to eighteen aspects of the present invention, the metal halide discharge lamp being supported by the illumination apparatus body.
The invention of the still additional aspect noted above is applicable to any of the apparatuses in which the metal halide discharge lamp according to any of the first to eighteenth aspects of the present invention is used for the illumination purpose. When it comes to a short arc type metal halide discharge lamp, the illumination apparatus of the present invention is suitable for use in illumination apparatus using the discharge lamp in combination with an optical system such as a reflector or a lens, e.g., a liquid crystal projector or an overhead projector, in a headlamp for a vehicle such as an automobile, and in illumination apparatus for shops such as an optical fiber illumination apparatus and a spot light.
On the other hand, when it comes to a long arc type metal halide discharge lamp, the discharge lamp of the present invention can be suitably used in various illumination apparatuses for the general illumination purposes such as a down light, an illuminating lamp mounted directly to the ceiling, an illumination apparatus for roads, an illumination apparatus for tunnels, in light projectors, and in a display apparatus.
To reiterate, the metal halide discharge lamp of the present invention is featured in that a halide of a metal which is unlikely to emit a visible light compared with a light emitting metal is sealed in the hermetic vessel in place of mercury. The particular metal halide is sealed in a high vapor pressure together with a halide of the light emitting metal. The particular construction defined in the present invention makes it possible to provide a metal halide discharge lamp which produces electric characteristics and light emitting characteristics substantially equal to those produced by the conventional metal halide discharge lamp having mercury sealed therein.
The present invention also provides a metal halide discharge lamp which produces at least one of the auxiliary effects a) to e) given below:
a) Good rising characteristics of spectral characteristics at the start-up time.
b) Capability of light control (dimming).
c) Small unevenness in characteristics.
d) Easy instant re-start up.
e) High resistance to rupture of hermetic vessel.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinbefore.